IEEE 802.11 wireless local area network (wireless LAN) is by far the most popular high-speed wireless communication network system today. IEEE 802.11 wireless local area network utilizes orthogonal frequency division multiplexing (OFDM) technique as a manor frequency modulation technique. OFDM introduces multi-carrier modulation technique to encode data bits into multiple sub-carriers. Unlike single carrier modulation system, in an OFDM system all frequencies are sent simultaneously in time. OFDM offers several advantages over single carrier modulation system in terms of better multipath effect immunity, simple channel equalization and relaxed timing acquisition constraints.
IEEE standard 802.11 prescribes the media access control (MAC) and physical (PHY) layers for a local area network wireless connectivity. In addition, IEEE standard 802.11 further defines three physical characteristics for wireless local area networks: diffused infrared, direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS), wherein the DSSS system has become a mainstream due to its high coding gain and robustness against interferences and noise. DSSS is a transmission technology used in wireless LAN system signal transmissions where a data signal at the transmitting station is combined with a higher data rate bit sequence, or chipping code, that divides the user data according to a spreading ratio. The chipping code is a redundant bit pattern for each bit that is transmitted, which increases the signal's resistance to interference. If one or more bits in the pattern are damaged during transmission, the original data can be recovered due to the redundancy of the transmission.
In a DSSS system, binary data bits are grouped into chunks and each chunk is mapped to a particular waveform termed a symbol which is transmitted across the channel after modulation with sub-carrier signal. These data symbols are transmitted over multiple channels by sub-carrier signals having orthogonal characteristics. However, if the sub-carrier frequency of digital receiver is not synchronous with the that of transmitter, the orthogonality of sub-carrier will vanish and severe inter-carrier interference (ICI) will be induced. In addition to the inter-carrier interference, the channel response of co-channel will interfere data symbols in a receiver. This co-channel interfering signals due to multi-path channel propagation is called inter-symbol interference (ISI). These crosstalk interferences are unwanted by-products in the middle of wireless communication, and the measurement and suppression of these interfering signals are the major goals that the wireless communication system designers are striving for.
Furthermore, for the current digital receiver used in a wireless communication system, it is feasible for processing modulated signals at one or two data rates only. When the modulated signals are transmitted through multi-path channels, there is a strong likelihood that portions of the modulated signals can not be accurately recovered to original data stream due to mismatch of demodulation units. According to IEEE 802.11 standard, in a DSSS system radio frequency signals at data rate of 5.5 or 11 Mbps are modulated by a coding technology called CCK (complementary code keying) modulation, which utilizes a series of codes called complementary sequences to multiply with original data stream. For radio frequency signals at data rate of 1 or 2 Mbps, the original data stream is multiplied by a spreading sequence or chipping code called barker code. Therefore, in order to recover the modulated signals to original data stream, an appropriate correlator must be employed in a receiver to remove spreading codes and retrieve original data stream. For example, a barker code correlator is used to synchronize the phase of a local barker spreading codes with received barker spreading codes in order to despread and recover data signal at ½ Mbps from a spread signal, whereas a CCK correlator is used to synchronize the phase of a local complementary codes with received complementary codes in order to recover data signal at 5.5 or 11 Mbps from a spread signal.
Accordingly, for the purpose of processing data signals at various data rates in a spread spectrum communication system, there is an inclination to provide a digital receiver capable of simultaneously processing modulated signals at various date rates, for example, 1, 2, 5.5 and 11 Mbps, with an embedded decision feedback equalizer (DFE) provided with ICI/ISI cancellation arrangement that can cope with channel-induced distortions.